Hydraulic system

ABSTRACT

The invention relates to a hydraulic system for actuating a belt-driven conical-pulley transmission with a variably adjustable transmission ratio, of a vehicle, having at least one hydraulic energy source and having a moment sensor that is supplied with working medium by a pump flow of the hydraulic energy source. 
     The invention is distinguished in that a disconnecting valve device is connected between the hydraulic energy source and the moment sensor, which makes it possible to connect or disconnect an additional pump flow of the hydraulic energy source, depending on need.

BACKGROUND OF THE INVENTION

The invention relates to a hydraulic system for actuating a belt-drivenconical-pulley transmission with a variably adjustable transmissionratio, of a vehicle, having at least one hydraulic energy source andhaving a moment sensor that is supplied with working medium by a pumpflow of the hydraulic energy source.

The object of the invention is to create a clutch control deviceaccording to the preamble of claim 1, with which losses that occur inoperating the hydraulic energy source can be reduced.

SUMMARY OF THE INVENTION

The problem is solved in a hydraulic system for actuating a belt-drivenconical-pulley transmission with a variably adjustable transmissionratio, of a vehicle, having at least one hydraulic energy source andhaving a moment sensor that is supplied with working medium by a pumpflow of the hydraulic energy source, by connecting a disconnecting valvedevice between the hydraulic energy source and the moment sensor thatmakes it possible to connect or disconnect an additional pump flow ofthe hydraulic energy source, if necessary. The first-named pump flow isalso referred to as the first pump flow. The other pump flow is alsoreferred to as the second pump flow. Independent of this identificationof the pump flows, however, the hydraulic energy source can include morethan two pump flows. The pump flows can be realized through a singlepump or through a plurality of pumps. Preferably, the pump flows areprovided by a single pump. If the second pump flow is connected to thefirst pump flow, then both pump flows are conveyed together to themoment sensor. If the second pump flow is disconnected, then the secondpump flow is conveyed into a working medium tank, so that only the firstpump flow is conveyed to the moment sensor. The invention furnishes asensible connection layout of a double-flow pump. That makes it possibleto uncouple a pump flow from the system pressure in certain operatingstates. On the other hand, the two pump flows provide for an adequatetransport volume in critical situations. Among other things, thatprovides the advantage that unnecessary losses in the transmission canbe reduced.

A preferred exemplary embodiment of the hydraulic system ischaracterized in that a check valve is connected between the two pumpflows in such a way that the first pump flow is severed from the secondpump flow as soon as the second pump flow is disconnected. That preventsthe first pump flow from being partially conveyed into the tank when thesecond pump flow is disconnected.

Another preferred exemplary embodiment of the hydraulic system ischaracterized in that the disconnecting valve device is connected via acontrol pressure line to a return line that comes from the momentsensor. The disconnecting valve device is actuated by way of the controlpressure line, depending on the pressure in the return line. Preferably,a clutch cooling valve device is connected into the return line betweenthe moment sensor and the control pressure line with which excessworking medium passes from the clutch cooling valve device to a clutchcooling device, for example by way of a jet pump.

Another preferred exemplary embodiment of the hydraulic system ischaracterized in that a pressure conversion valve device is connectedinto the control pressure line. The pressure conversion valve deviceenables the actuating force at the disconnecting valve device to beincreased significantly.

Another preferred exemplary embodiment of the hydraulic system ischaracterized in that the disconnecting valve device is executed as a2/2 directional valve with an open position and a closed position, intowhich the 2/2 directional valve is pre-tensioned. When the disconnectingvalve device is in the open position, the second pump flow is conveyedinto the tank; that is, it is disconnected. When the disconnecting valvedevice is in the closed position, the second pump flow is connected tothe first pump flow. Both pump flows are then conveyed together to themoment sensor. The pre-tensioning is realized for example with the aidof a pre-tensioning spring device.

Another preferred exemplary embodiment of the hydraulic system ischaracterized in that the disconnecting valve device is connected viaanother control pressure line to a connecting valve device, which isconnected ahead of the moment sensor. The connecting valve device, whichis preferably connected between the pressure regulating valve device andthe moment sensor, works together with the pre-tensioning of thedisconnecting valve device, in order to quickly switch the disconnectingvalve device to its closed position when necessary.

Another preferred exemplary embodiment of the hydraulic system ischaracterized in that the connecting valve device is executed as a 3/2directional valve with an open position and a closed position, intowhich the 3/2 directional valve is pre-tensioned. When the connectingvalve device is in the open position, the other control pressure line ispressurized with the pressure ahead of the moment sensor. When theconnecting valve device is in the closed position, the other controlpressure line is relieved into the tank.

Another preferred exemplary embodiment of the hydraulic system ischaracterized in that the connecting valve device is actuated with thepressure ahead of the moment sensor. If the pressure ahead of the momentsensor rises above a specified value, the connecting valve device isopened and the disconnecting valve device is closed, in order to connectthe second pump flow again.

Another preferred exemplary embodiment of the hydraulic system ischaracterized in that the disconnecting valve device is pressurizablevia the other control pressure line and the connecting valve device withthe pressure ahead of the moment sensor. That enables the second pumpflow to be connected quickly when necessary.

Another preferred exemplary embodiment of the hydraulic system ischaracterized in that a pressure regulating valve device is connectedbetween the hydraulic energy source and the moment sensor. The pressureregulating valve device, which is also referred to as a pressure holdingvalve device, provides for a desired system pressure to be maintainedahead of the moment sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and advantages of the present invention willbecome further apparent upon consideration of the following description,taken in conjunction with the accompanying drawings in which:

FIG. 1: a detail of a hydraulic circuit diagram of a hydraulic systemaccording to the invention, according to a first exemplary embodiment,having a disconnecting valve device and a connecting valve device, and

FIG. 2: a detail of a hydraulic circuit diagram similar to that in FIG.1 according to a second exemplary embodiment, having an additionalpressure conversion valve device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 depict hydraulic systems 1 and 51, respectively, with theaid of symbols conventionally used in hydraulics for a hydraulic circuitdiagram. In the hydraulic circuit diagram, a tank with hydraulic mediumis designated at each of various places by a symbol with the referencelabel 5. The hydraulic medium contained in the tank is preferablyhydraulic oil, also referred to as oil. Hydraulic system 1, 51 serves tocontrol a belt-driven conical-pulley transmission, which is situated inthe power train of a motor vehicle. Such belt-driven conical-pulleytransmissions are also referred to as CVT transmissions (CVT,continuously variable transmission). This is a stepless vehicletransmission, which makes great driving comfort possible throughjerk-free changing of the transmission ratio.

The two hydraulic systems 1, 51 are so similar that the same referencelabels are used to designate like parts. In contrast to a conventionalhydraulic system, hydraulic systems 1, 51 include two pump flows 2, 3,which are provided by a hydraulic energy source 4. Hydraulic energysource 4 is preferably a hydraulic pump which conveys the two pump flows2, 3 out of tank 5 into a connecting line 6. From connecting line 6comes another connecting line 7, through which hydraulic system 1, 51 isconnected to other consumers, which are not shown for the sake ofclarity. The other consumers are for example disk sets of thebelt-driven conical-pulley transmission, clutches, cooling devices,etc., which act in combination through a plurality of valve devices.

The two pump flows 2, 3 are connected via connecting line 6 to a momentsensor 10, which ensures that when the transmission is in operationthere is constantly sufficient clamping pressure applied to transmittorque between the pulleys and a corresponding encircling element of thebelt-driven conical-pulley transmission, in particular depending on thetorque applied to the belt-driven conical-pulley transmission. Thepressure ahead of moment sensor 10 is identified as p_(MF). The flowrate of hydraulic medium through or after moment sensor 10 is identifiedas Q_(nMF). Connected ahead of moment sensor 10 is a pilot valve device12, which is also referred to as a pressure regulating valve device, andwhich serves to keep the pressure p_(MF) ahead of moment sensor 10 to aminimum pressure level of 6 to 6.5 bar, for example. Pressure regulatingdevice 12 is actuatable through pressure return lines 14, 15.

Medium sensor 10 is connected through a return line 18 to an input line20, which in turn is connected to the inputs of pump flows 2, 3.Connected to input line 20 ahead of pump flows 2, 3 is a pump injector21, which serves to improve the suction performance. Situated in returnline 18 and connected after moment sensor 10 is a clutch cooling valvedevice 24, which serves to convey surplus hydraulic medium from returnline 18 through a jet pump 25 to a clutch cooling device (not shown).Return line 18 is therefore also referred to as the cooler return line.

Clutch cooling valve device 24 is connected after a pressure holdingvalve device 28, which serves to maintain a desired minimum pressure inreturn line 18. Hydraulic medium is discharged from return line 18 intotank 5 through a discharge line 30. A substitute restrictor 31 issituated in discharge line 30. In reality there are numerous restrictorssituated in return line 18 between the output of moment sensor 10 andpressure holding valve device 28, for example for pulley cooling, foroiling and so on. These restrictors (not shown) are combined insubstitute restrictor 31 for the sake of simplification.

Disconnection of second pump flow 3 is only possible if moment sensor 10is supplied with adequate flow volume. Assuming the critical conditionsin regard to temperature and gap height for a certain flow volumeQ_(nMF) through moment sensor 10, a maximum possible pressure p_(MF)exists ahead of moment sensor 10. Second pump flow 3 can only bedisconnected if first pump flow 2 is conveying sufficient volume tobuild up the necessary pressure p_(MF) ahead of moment sensor 10.

In conjunction with the present invention, a threshold value for Q_(nMF)of about 3 liters per minute has proven to be especially advantageous.Starting at a flow volume Q_(nMF) of about 3 liters per minute,according to an aspect of the present invention second pump flow 3should be disconnected, so that for large portions of driving operationone pump flow conveys or is conveyed into the tank, and thus unwantedlosses are minimized. It has also been found in conjunction with thepresent invention that with this flow volume a pressure p_(MF) of onlyabout 30 bar can be built up ahead of moment sensor 10. According toanother aspect of the invention, this pressure affects the circuitaccording to the invention.

The area between the output of moment sensor 10 and pressure holdingvalve device 28 has proven to be especially well suited for obtaininginformation about the flow volume Q_(nMF) via moment sensor 10. If thepressure p_(MF) ahead of moment sensor 10 is less than 1.8 bar, thenpressure holding valve device 28 is closed, and only the flow volumeQ_(nMF) which is flowing through moment sensor 10 determines thepressure in this area. This pressure can be determined with the aid of arestrictor formula. A flow volume of 2 liters per minute results in apressure p_(R) of about 0.5 bar in return line 18 after moment sensor 10or after clutch cooling valve device 24. When this pressure is reached,then according to an essential aspect of the invention a disconnectingvalve device 33 is supposed to switch second pump flow 3 in thedirection of tank 5.

Disconnecting valve device 33 is connected to the outputs of the twopump flows 2, 3 of hydraulic energy source 4 through a disconnectingline 34. A change-back valve device 35 is connected into disconnectingline 34 between the outputs of the two pump flows 2, 3. Disconnectingvalve device 33 is designed as a 2/2 directional valve that has a closedposition in which a connection between disconnecting line 34 and tank 5is interrupted. Disconnecting valve device 33 is pre-tensioned by aspring 36 in its closed position depicted in FIGS. 1 and 2. In an openposition (not shown), disconnecting valve device 33 connectsdisconnecting line 34 to tank 5.

Disconnecting valve device 33 is actuated by way of a control pressureline 37, through which disconnecting valve device 33 is pressurized withthe pressure p_(R) behind moment sensor 10 or behind clutch coolingvalve device 24. The control pressure provided via control pressure line37 counteracts the pre-tensioning force of spring 36. Above a certainpressure in return line 18, which acts on disconnecting valve device 33through control pressure line 37, disconnecting valve device 33 or acorresponding valve piston in disconnecting valve device 33 moves fromthe depicted closed position into the open position; that is,disconnecting valve device 33 begins to open. That causes part of theflow volume of second pump flow 3 to be conveyed in the direction of thetank.

The remaining portion of the flow volume of second pump flow 3, on theother hand, continues to be conveyed through disconnecting line 34 tomoment sensor 10. At disconnecting valve device 33 an equilibriumdevelops between the force from the pressure p_(R) in return line 18 orcontrol pressure line 37 and the pre-tensioning force of spring 36. Ifthe volumetric flow rises further, then at some time the flow volume offirst pump flow 2 is sufficient to supply the additional consumers viaconnecting line 7, and in addition to also transport the desired 3liters per minute through moment sensor 10 that are necessary to switchdisconnecting valve device 33. At that moment second pump flow 3 istransporting entirely in the direction of tank 5, and check valve 35closes. The pressure against which second pump flow 3 must work whentransporting then collapses suddenly, and the losses of hydraulic energysource 4 are greatly reduced.

In most cases a flow volume of 3 liters per minute is sufficient formoment sensor 10 to function. However, at higher torques andconsequently higher p_(MF) levels the volume flow requirement isgreater. That need can be met by adding second pump flow 3 back in abovea certain pressure level of p_(MF). According to another aspect of theinvention, such connecting of second pump flow 3 is achieved by means ofa connecting valve device 40.

It has been found in connection with the present invention that for thedesign of the circuit according to the invention the switching point ofpressure holding valve device 28 is relevant at around 5.9 liters perminute. From that point on the pressure p_(R) no longer has anyrestriction behavior but remains nearly constant, since pressure holdingvalve device 28 operates as an absolute pressure valve. The ratio ofareas between the effective area of pressure p_(MF) and of pressurep_(R) at disconnecting valve device 33 is chosen according to anotheraspect of the invention so that at a break point the maximum p_(MF)causes second pump flow 3 to be connected. If the pressure exceeds aswitch-on threshold, then disconnecting valve device 33 is switched backto its closed position again, so that the flow volume of both pump flows2, 3 is available.

It has also been found in connection with the present invention that adirect action of the pressure p_(MF) on the disconnecting valve devicecould result in even a very low pressure p_(MF) being sufficient toreconnect the second pump flow 3. It would be desirable, however, toreconnect second pump flow 3 only at higher pressures. At the same time,however, it is important to ensure that the second pump flow isreconnected in any case before the maximum possible pressure p_(MF) isreached. By adding the second pump flow only when pressure becomesgreater, it is possible to reduce the losses in particular whentraveling at high velocity, that is, at high rotational speeds or highflow volumes.

For this purpose a connecting valve device 40 is provided, which isdesigned as a 3/2 directional valve. Connecting valve device 40 ispre-tensioned in its closed position by a spring 41, and is connected tothe input of moment sensor 10 via a moment sensor pressure line 42, sothat the pressure p_(MF) prevails in moment sensor pressure line 42. Inthe closed position of connecting valve device 40 depicted in FIGS. 1and 2, another control pressure line 38, which comes from disconnectingvalve device 33, is connected through a tank line 43 to tank 5, so thatthe other control pressure line 38 is relieved into tank 5. A connectionbetween the other control pressure line 38 and the moment sensorpressure line 42 is interrupted in the closed position by connectingvalve device 40.

In its closed position (not shown), connecting valve device 45 connectsthe moment sensor pressure line 42 to the other control pressure line38, so that the pressure p_(MF) ahead of the moment sensor acts ondisconnecting valve device 33 through the moment sensor pressure line 42and the other control pressure line 38. Connecting valve device 40 ispressurized against the pre-tensioning force of spring 41 with thepressure p_(MF) through a moment sensor control pressure line 44 . Whenthe pressure p_(MF) rises above a specified value, then connecting valvedevice 40 opens, so that disconnecting valve device 33 is closed, sincethe control pressure provided via the other control pressure line 38works together with the pre-tensioning force of spring 36.

In the hydraulic system 1 depicted in FIG. 1 there can be a problem inthat the actuating force for disconnecting valve device 33 throughcontrol pressure line 37 is rather small; that is, the pressure for aswitching action is about 0.5 bar. A possible result of that is that anunwanted effect of flow forces at disconnecting valve device 33 can nolonger be ignored.

As a remedial measure, in FIG. 2 a pressure conversion valve device 60is provided in control pressure line 37 in hydraulic system 51. Pressureconversion valve device 60 can be pressurized at an elevated pressurethrough a setting pressure line 62. Pressure conversion valve device 60is actuated by the pressure in control pressure line 37 through apressure return line 64. Pressure conversion valve device 60 serves toelevate the pressure in return line 18 or in control pressure line 37,and thus to raise the actuating force at disconnecting valve device 33significantly.

Although particular embodiments of the present invention have beenillustrated and described, it will be apparent to those skilled in theart that various changes and modifications can be made without departingfrom the spirit of the present invention. It is therefore intended toencompass within the appended claims all such changes and modificationsthat fall within the scope of the present invention.

1. A hydraulic system for actuating a belt-driven conical-pulleytransmission with a variably adjustable transmission ratio, of avehicle, having at least one hydraulic energy source and having a momentsensor that is supplied with working medium by a pump flow of thehydraulic energy source, characterized in that a disconnecting valvedevice is connected between the hydraulic energy source and the momentsensor, which makes it possible to connect or disconnect an additionalpump flow of the hydraulic energy source, depending on need.
 2. Thehydraulic system according to claim 1, characterized in that a checkvalve is connected between the two pump flows in such a way that thefirst pump flow is severed from the second pump flow as soon as thesecond pump flow is disconnected.
 3. The hydraulic system according toclaim 1, characterized in that the disconnecting valve device isconnected via a control pressure line to a return line that comes fromthe moment sensor.
 4. The hydraulic system according to claim 3,characterized in that a pressure conversion valve device is connectedinto the control pressure line.
 5. The hydraulic system according toclaim 3, characterized in that the disconnecting valve device isexecuted as a 2/2 directional valve with an open position and a closedposition, into which the 2/2 directional valve is pre-tensioned.
 6. Thehydraulic system according to claim 3, characterized in that thedisconnecting valve device is connected via another control pressureline to a connecting valve device that is connected ahead of the momentsensor.
 7. The hydraulic system according to claim 6, characterized inthat the connecting valve device is executed as a 3/2 directional valvewith an open position and a closed position, into which the 3/2directional valve is pre-tensioned.
 8. The hydraulic system according toclaim 6, characterized in that the connecting valve device is actuatedwith the pressure ahead of the moment sensor.
 9. The hydraulic systemaccording to claim 6, characterized in that the disconnecting valvedevice is pressurizable with the pressure ahead of the moment sensor viathe other control pressure line and the connecting valve device.
 10. Thehydraulic system according to claim 1, characterized in that a pressureregulating valve is connected between the hydraulic energy source andthe moment sensor.